For a short-throated flume to accurately measure flow, it is desirable to keep the flume’s dimensions with +/-2% of nominal. But what happens when they aren’t? For Rectangular Cutthroat Flumes, investigations have shown that corrects for throat width can be made.
Variations in a flume’s throat width would normally mean that the flume would over or under represent the actual flow rate. However, for Rectangular Cutthroat flumes a free flow discharge correction factor (Ktf) and a submerged flow discharge correction factor (Kts) can be calculated.
To determine the correction factor (Ktf or Kts) for a Rectangular Cutthroat flume, all that needs to be determined is the mean throat width (Wf). The mean throat width is obtained by simply taking a number of measurements of the throat width from the floor to the top of the flume (W).
Typically the first measurement is taken 1/2-inches above the floor of the flume and then equally spaced measurements are taken from there to the top of the flume. The initial measurement is taken above the floor, as there is always some bend or lamination radius where the vertical sidewalls of the flume meet the floor.
The mean throat width (Wf) is then calculated by:
Wf = (W1 + W2 + W3 +…. + W) / N
Thus,
Ktf = Wf / Wspec
Where,
Ktf = free flow discharge correction factor
Wf = mean throat width from 1/2-inch above the flume floor to the top of the flume
Wspec = standard throat width for the flume
N = number of throat width measurements
With the above, the free flow discharge equation becomes:
Q(f) = Ktf C H^n
Example:
A Cutthroat flume has a specified throat width of 12-inches, a length of 54-inches, and a standard sidewall height of 1/3 the length = 18-inches.
Several measurements of the throat are taken and are found to be:
W1 = 12.025-inches
W2 = 12.102-inches
W3 = 12.017-inches
Wf = (W1 + W2 + W3) / N
Wf = (12.025 + 12.102 + 12.017) / 3 = 12.048-inches
Kf = 12.048-inches / 12-inches = 1.004
The free flow equation becomes:
Q(f) = 1.004 C H^n
